1. Field of the Invention
The present invention relates to an image processing apparatus and a method for processing image information acquired by an image reading apparatus configured to perform spectrometric scanning to optically read color image information from a document. More specifically, the present invention relates to a method for determining a color blur pixel as an achromatic color when an achromatic edge portion is optically read from a color document.
2. Description of the Related Art
FIGS. 1 and 2 illustrate two representative arrangements of the image reading apparatus configured to perform spectrometric scanning to optically read color image information from a document.
FIG. 1 illustrates an internal configuration of an image reading apparatus including a reducing optical system.
A document 12 can be placed on a document positioning plate 11, which is made of glass. A light source unit 15 includes a light source 13 and a reflection mirror 14, which are mechanically fixed to the casing of the light source unit 15. The light source unit 15 can move in a direction indicated by an arrow P. The light source unit 15, the incorporated light source 13 and the reflection mirror 14 have a width equivalent to or longer than the width of the document 12 to be read (i.e., the size in a direction perpendicular to the drawing surface of FIG. 1).
An arrow Q indicates the optical path of light emitted from the light source 13. The light emitted from the light source 13 illuminates the document 12 via the document positioning plate 11. Reflection light according to a spectral reflectance of the document 12 is incident on the reflection mirror 14. The reflection mirror 14 changes the optical path of the reflection light so that the reflection light can pass through a reducing lens 16. The reducing lens 16 can reduce the reflection light having a width equivalent to or wider than the document 12. The reflection light reduced by the reducing lens 16 reaches a photoelectric conversion element 17.
The photoelectric conversion element 17 is, for example, a charge-coupled device (CCD) sensor or a complementary metal-oxide semiconductor (CMOS) sensor, which is a semiconductor element capable of converting input light information into an electric signal. The photoelectric conversion element 17 has a width narrower than that of the document 12. The reducing lens 16 has a reduction rate determined considering a ratio of the width of the document 12 to the width of the photoelectric conversion element 17.
The light source unit 15 and the reducing lens 16 synchronously move in directions indicated by arrows P and T to scan the document 12, while the photoelectric conversion element 17 generates electronic data.
FIG. 2 illustrates an example internal configuration of an image reading apparatus including an equal magnification optical system.
A contact image sensor (CIS) unit 22 includes a light source 13, an equal magnification lens 21, and a photoelectric conversion element 23, which are mechanically fixed to the casing of the CIS unit 22. The CIS unit 22 moves in a direction indicated by an arrow U. The photoelectric conversion element 23 is, for example, a CCD sensor or a CMOS sensor, which is a semiconductor element capable of converting input light information into an electric signal. The CIS unit 22, the incorporated light source 13, the equal magnification lens 21, and the photoelectric conversion element 23, has a width equivalent to or longer than the width of the document 12 to be read (i.e., the size in a direction perpendicular to the drawing surface of FIG. 1). An arrow V indicates the optical path of light emitted from the light source 13.
The light emitted from the light source 13 illuminates the document 12 via the document positioning plate 11. Reflection light according to a reflectance of the document 12 is incident on the equal magnification lens 21. The incident light has a width equivalent to or wider than the width of the document 12 and reaches the photoelectric conversion element 23 via the equal magnification lens 21. The CIS unit 22 moves in a direction indicated by an arrow U to scan the document 12 while photoelectric conversion element 22 generates electronic data.
In general, an optical system usable for a method for generating electronic data of colors includes a light source serving as the light source 13, which can generate light having a wavelength in the visible range. Furthermore, the optical system includes color separation filters of red (R), green (G), and blue (B), i.e., three primary colors, associated with three photoelectric conversion element arrays (line sensors) respectively, which can serve as the photoelectric conversion element 17 or 23. The optical system can be used to scan the document 12 and generate color electronic data by combining RGB pixel signals obtained by respective line sensors.
FIG. 3 illustrates a representative configuration of the photoelectric conversion element 17.
The photoelectric conversion element 17 includes three line sensors dedicated to R, G, and B colors, i.e., a line sensor (R) 31, a line sensor (G) 32, and a line sensor (B) 33, which can detect three (R, G, B) color components to capture a composite color image. Each line sensor includes a predetermined number of pixels arrayed in a main scanning direction, which constitute a light-receiving portion for reading an image of a document with a predetermined resolution.
When the resolution is 600 dpi, an area of approximately 42 μm×42 μm on a document corresponds to one pixel on electronic data.
When these line sensors are disposed on a semiconductor, a distance equal to or greater than 0 μm is required between two line sensors. When the distance between two line sensors is 0 μm, the delay amount is equal to one line that corresponds to the distance between the centroids of two line sensors. If the distance is larger than 0, the delay amount is equal to n lines (n=integer) that is the distance between two lines projected on a document.
FIG. 4 illustrates an example RGB image capturing method in a case where the delay amount is n=2. While the light source unit 15 or the CIS unit 22 is moving in the sub scanning direction, the line sensor (R) 31 reads a target line M=m at time T=t. In this case, the line sensor (G) 32 reads the (m−2)th line and the line sensor (B) 33 reads the (m−4)th line. After a unit time has elapsed, i.e., at time T=t+1, the line sensor (R) 31 reads the (m+1)th line, the line sensor (G) 32 reads the (m−1)th line, and the line sensor (B) 33 reads the (m−3)th line. The photoelectric conversion element 17 repeats the similar operation and completes reading R, G, and B data from the target line M=m at time T=t+4 after starting the operation at time T=t. During this period (T=t to T=t+4), i.e., before completing the reading of the B line data, a line buffer (not illustrated) holds the R and G line data. When the reading of the R, G, and B data is completed at the time T=t+4, a color image of the target line M=m can be obtained.
The above-described description can be applied similarly to any other case where the delay amount is not n=2, although the time required to obtain R, G, and B data of a target line is variable depending on the delay amount. The capacity of the line buffer is required to be changed correspondingly.
To accurately capture a color image of a document as electronic data, it is important to synchronize the timing of the photoelectric conversion element 17 or 23 storing electric charge for each line with a moving amount of the light source unit 15 or the CIS unit 22 in the sub scanning direction.
For example, the above-described movement of the light source unit 15 or the CIS unit 22 in the sub scanning direction can be realized by a stepping motor. When the stepping motor is used, the image reading apparatus is not free from color misregistration randomly appearing in the sub scanning direction, which is a phenomenon occurring due to various factors such as irregularity in rotation, vibration in a driving/transmission mechanism (e.g., gears and belts), and vibration or disturbance at other frictional portion.
The color misregistration refers to deviations among reading lines of the line sensor (R) 31, the line sensor (G) 32, and the line sensor (B) 33.
The color misregistration may also occur when the delay amount between the lines is extremely large. For example, when n=12, the difference in reading time between R and B data is equivalent to the time corresponding to 24 lines. During such a long time, variations in the image reading state caused by the above-described mechanical factors tend to become significantly larger.
From the foregoing, to accomplish the color image reading operation accurately, it is desired to read R, G, and B data of a target line within a short time using an accurate driving mechanism. However, the cost increases if the driving mechanism is required to attain a higher reading accuracy.
FIG. 5 illustrates R, G, and B luminance level signals of a target pixel obtained when the photoelectric conversion element 17 performs reading processing on a document including a thin black line (including a black character) area in contrast with a white background.
In FIG. 5, dotted lines indicate sampling lines along which the line sensor (R) 31, the line sensor (G) 32, and the line sensor (B) 33 perform document reading processing. If no color misregistration occurs in the document reading operation, the luminance levels of R, G, and B are identical to each other. In such a case, when the sensor reads a boundary between white and black areas (i.e., an edge portion) on a document, the sensor generates a signal representing a halftone gray similar to an achromatic color. When the sensor reads the fifth line or a subsequent line, the sensor generates a signal representing a gray having a lower luminance. FIG. 6 illustrates electronic data obtained by the reading operation illustrated in FIG. 5.
The electronic data obtained in this manner can be used in a chromatic/achromatic color determination described below, according to which the target pixel on the fourth line can be identified as an achromatic color. FIG. 7 illustrates R, G, and B luminance level signals of a target pixel obtained when the color misregistration is present in the sub scanning direction. According to the example illustrated in FIG. 7, sampling timing of the line sensor (R) 31 is one line earlier than the sampling timing of the line sensor (G) 32 or the line sensor (B) 33 in the vicinity of an edge portion on the document.
In this case, the luminance levels of R, G, and B on the third line and the fourth line are not identical to each other. The luminance level of the R channel is constantly lower than the luminance levels of other two channels. FIG. 8 illustrates electronic data obtained by the reading operation illustrated in FIG. 7, in which each of the third line and the fourth line is a saturated halftone color, i.e., a color blur.
The color blur at an achromatic edge portion caused by the color misregistration phenomenon deteriorates the quality of image data read by the line sensors or weakens the “auto color select (ACS)” function usable to determine whether a document is a color document.
As discussed in Japanese Patent Application Laid-Open No. 2000-151937, there is a conventional method for performing achromatic color determination on a color blur pixel caused by the color misregistration. The method includes extracting an edge from a Y(0.25R+0.5G+0.25B) signal representing the luminance, determining an achromatic area from an RGB signal, determining an edge and achromatic area as a thin black line or a black character region, and performing achromatization on the determined area.
However, the above-described conventional method includes the problems described next.
It is now assumed that an R signal, a G signal, and a B signal deviate from each other as illustrated in FIG. 19 due to the color misregistration caused by a scanner when reading a document including a black area with a while background. In this case, reading of the R signal is advanced. The level of the R signal greatly decreases at the third line while the G signal and the B signal read the white background. The electronic data obtained by combining the R signal with the G and B signals has a value indicating a bright cyan. As described above, when the signal levels of three channels are not identical to each other, the white area cannot be identified as an achromatic area. Hence, this area cannot be determined as a thin black line or a black character region, and cannot be achromatized.